Field of the Invention
The present invention relates to a developing apparatus that develops an electrostatic latent image formed on a photoconductor drum, to a process cartridge that forms a toner image and that is attachable/detachable to/from the apparatus body of an image forming apparatus, and to an image forming apparatus that relies on electrophotography.
Description of the Related Art
In electrophotographic image forming apparatuses relying on electrophotography, a photoconductor drum and a process means that acts on the photoconductor drum may be configured together in the form of an integrated process cartridge. The process cartridge can be attached/detached to/from the apparatus body of the image forming apparatus. Such process cartridge schemes are more convenient in that the user his/herself can service the image forming apparatus, without depending on a service man. Accordingly, process cartridge schemes have come to be widely used in image forming apparatuses.
The process cartridge is provided with a developing apparatus that develops an electrostatic latent image formed on the photoconductor drum. The developing apparatus supplies toner to the electrostatic latent image formed on the photoconductor drum, as a result of which the electrostatic latent image becomes developed in the form of a toner image. Schemes for developing electrostatic latent images on photoconductor drums include jumping development schemes. In a jumping development scheme, magnetic toner is caused to fly through change of the electric field between the photoconductor drum and a developing roller. Specifically, magnetic toner is caused to fly by finely modifying the strength of the electric field. In a jumping development scheme, toner degradation can be suppressed since the photoconductor drum and the developing roller do not come into contact with each other, and toner is not rubbed between the photoconductor drum and the developing roller.
A demand has arisen in recent years towards reducing the amount of toner that is consumed in order to form an image. Specifically, an identical amount of toner is required to allow printing a larger number of images. Doing so allows reducing the size of the container in which toner is held, and, in consequence, reducing the size of the image forming apparatus. In a jumping development scheme, as is known, a substantial amount of toner adheres to edge portions of electrostatic latent images on the photoconductor drum.
In consequence, the toner consumption amount tends to increase when images are formed that include numerous edges, for instance characters and fine lines. Herein FIGS. 9A and 9B are diagrams for explaining conventional jumping development. In some conventional instances, toner on the developing roller is immobilized in the form of “bristles” on account of the magnetic force of a magnet that is disposed inside the developing roller, as illustrated in FIG. 9A (hereafter, such “bristles” will be referred to as magnetic brush). The toner consumption amount at the edge portions is greater herein, since the entire magnetic brush on the developing roller becomes adhered, as it is, on the edge portions of the electrostatic latent image.
Therefore, in the technology disclosed in Japanese Patent No. 4532996, in order to reduce the toner consumption amount at edge portions, toner particles on the developing roller are not caused to move in the form of a magnetic brush, but separately as individual particles. A state in which toner particles on the developing roller are in the form of separate individual particles referred to as a cloud state. In the technology disclosed in Japanese Patent No. 4532996, the toner particles on the developing roller are brought to a cloud state, as illustrated in FIG. 9B, and the toner consumption amount at the edge portions of the electrostatic latent image is reduced as a result.
In the technology disclosed in Japanese Patent No. 4532996, however, fogging occurs when the process speed of the image forming apparatus is increased. To bring the toner particles on the developing roller to a cloud state in jumping development, a magnetic constraining force that the magnet within the developing roller exerts on the toner is made weaker, to bring about thereby a cloud state. The toner particles in the form of a cloud state move reciprocally between the photoconductor drum and the developing roller, and the electrostatic latent image on the photoconductor drum becomes developed as a result.
FIGS. 10A and 10B are diagrams for explaining the cause of fogging in jumping development. Flow of air occurs between the photoconductor drum and the developing roller due to rotation of the photoconductor drum and the developing roller. Such air flow does not affect the cloud-state toner when the process speed of the image forming apparatus is low and the rotational speed of the photoconductor drum and of the developing roller is low.
When the rotational speed of the photoconductor drum and of the developing roller increases, however, the influence on the cloud-state toner increases likewise. Individual toner particles have smaller mass than the magnetic brush, and hence toner particles in a cloud state are more affected by air flow than toner particles in a magnetic brush state.
Cloud-state toner particles that move reciprocally between the photoconductor drum and the developing roller move downstream, in the rotation direction of the photoconductor drum and the developing roller, on account of the air flow between the photoconductor drum and the developing roller. As a result, reciprocally moving toner particles that should return to the developing roller may in some instances fail to do so. The toner particles that do not return to the developing roller appear on the image in the form of fogging.